lnwallet: remove need for lnwallet to have access to the private key

This paves the way for lnd to work with hardware wallets, in which case it will not have access to the private key.

mock.go

@@ -222,8 +222,8 @@ func (m *mockWalletController) NewAddress(addrType lnwallet.AddressType,
 		m.rootKey.PubKey().SerializeCompressed(), &chaincfg.MainNetParams)
 	return addr, nil
 }
-func (*mockWalletController) GetPrivKey(a btcutil.Address) (*btcec.PrivateKey, error) {
-	return nil, nil
+func (*mockWalletController) IsOurAddress(a btcutil.Address) bool {
+	return false
 }
 
 func (*mockWalletController) SendOutputs(outputs []*wire.TxOut,